The present invention relates to dynamoelectric machines and, more particularly, to induction motors having a fixed polar configuration (i.e., a fixed number of main or running poles and therefore only one no-load synchronous speed) that are particularly adapted for operation in conjunction with a driven device such as a fan, and wherein different operational speeds for a given driven device load are obtained by selectively changing the field strength (e.g., by changing the number of effective ampere turns, or by changing the impressed voltage) without changing the number of poles.
A very large number of single phase motors are sold each year in the horsepower range of about 1/20 to 3/4 horsepower for air moving fan applications, in many of which the fan is driven directly by the motor. As a consequence of such direct drive, and the speed-torque characteristics of a fan, these applications normally require relatively little starting torque. For this reason, shaded pole induction motors and permanent split capacitor induction motors (which have low starting torque) have found wide usage. This relatively wide usage has resulted because of the relative simplicity in design and fabrication of such motors. This simplicity (at least in part) is due to the fact that these types of motors do not require removal of a start winding from the operating motor circuit at operating speeds.
Conventional shaded pole motors generally are handicapped by cooling limitations that result from the low efficiencies associated with such motors. On the other hand, permanent split capacitor motors tend to be handicapped by the expense associated with start windings that usually have many turns of small wire and a continuous duty capacitor.
The relatively low efficiency of shaded pole motors is due to, among other things, relatively high losses in the rotor and shading coils, these losses being manifest of course in the form of heat. When more heat is generated by a motor, it becomes more difficult to provide adequate motor cooling. If the rotor and/or shading coil losses of a shaded pole motor could be reduced, it should be possible to make other advantageous cost reducing changes while still serving the market place with equal effectiveness.
Since the direct drive fan market is primarily interested in motor temperature rise; in those cases where the temperature rise of a presently existing motor is acceptable, a decrease in rotor loss would allow greater loss in the main winding and associated stator core losses. This would mean that current densities could be raised in the main winding with consequent material reductions and cost savings--both for shaded pole and permanent capacitor motors.
In those cases where shaded pole motors are not now being used because the temperature rise of presently available motors are too high, a newly designed shaded pole motor with a decrease in rotor losses (without an increase in main coil and core losses) could allow such a shaded pole motor to be applied. The less expensive shaded pole motors then could replace the more expensive permanent capacitor motor type in many cases.
It thus will be understood that innovation which would provide such advantageous alternatives would be very desirable. It would of course be of value to reduce the rotor losses in all types of motors having relatively high space harmonic content in the stator magnetic field; e.g., in motors having only a few (e.g., one or two) concentric coils per coil group (or pole). This is usually the case, for example, in relatively small permanent capacitor motors (e.g., about six inches or less diameter) having a relatively large number of poles (e.g., four or six or more) and in shaded pole motors.
For direct fan or blower drive applications, shaded pole or permanent capacitor motors are normally designed to have a fixed number of poles. The pole number is selected so that the synchronous speed of the motor under no load conditions will be somewhat greater than the highest desired operating speed of the fan that is to be driven by the motor. For example, if the highest desired fan speed is in the neighborhood of 1000 rpm, a six pole motor would probably be selected provided a 60 HZ voltage source was to be used (it being understood that the synchronous speed of a six pole induction motor energized by a 60 HZ source or 50 HZ source is about 1200 rpm or 1000 rmp, respectively).
Whether of the shaded pole or permanent capacitor type, motors selected for multi-fan speed operation are usually designed to have taps coming from the main windings. Different speeds then occur (under load) when the slip is varied by varying the field strength. The field strength is varied of course by the selective energization of different taps or winding leads. The highest desired speed will occur when the fewest number of turns are placed across the line voltage. On the other hand, when all of the turns are energized by line voltage, the field strength, and thus speed under a given load will be a minimum. It is again emphasized that multi-speed operation of the shaded pole and permanent capacitor motors discussed herein is obtained by changing the field strength (by changing the number of effective winding turns or by changing the applied voltage), rather than by changing the number of poles that are energized. These types of motors have long been recognized as having an unstable low-speed connection. "Unstable low-speed" is described in more detail hereinbelow, but it is now noted that the low speed of such motors is relatively sensitive to changes in applied voltage, fan or blower inlet and outlet restrictions, and so forth.
The voltage supplied to a residence may be greater or less than nominal (thus causing objectionable low-speed motor instability) as a result of power transmission line voltage drops, peak loading of a gnerating station, or what is currently known as "brown-outs".
For these and other reasons, original equipment manufacturers who purchase variable-field strength multi-speed motors (hereinafter referred to simply as "multi-speed" motors) for direct drive fan or blower applications are usually interested in the speed torque relationships of such motors not only at the nominal voltage but also at voltages that vary from the nominal voltage (for example, plus or minus 10 percent).
Heretofore, it has been necessary for motor manufacturers and purchasers of multi-speed motors to compromise between a motor design having relatively high efficiency at high speed but poor low-speed stability, or a motor designed to have good low-speed stability but poor high speed efficiency. The compromise solution has usually involved the selection of a motor design having relatively poor low-speed stability and less than maximum efficiency.
It thus should be understood that it would be particularly desirable to provide new and improved induction motors having improved efficiencies; and, depending on the intended application of the motor, it would be particularly advantageous if such motors would have improved low-speed stability when designed for multi-speed applications.